Monster power levels are much more relative in 4E; you can take a monster concept and create it at basically any level. In 3.5E, I think the reason Changelings were created was to solve the problem that Doppelganger stats didn't make for very good PCs on account of their advanced power level and the ECL debacle. Mechanically, both creatures do pretty much the same things, except that the Doppelganger can read minds. Thus, if you consider the Changeling to be basically a +0 LA version of the Doppelganger, then you'll see that there's not much difference between the two races aside from power level. Since power level is very relative in 4E, the two races would have been 95% redundant with one another in the new edition.
As for why the 4E Doppelgangers look like 3.5E's Changelings, I have no idea -- to make them look less "alien", and thus more suitable as a player race, perhaps?
I wonder what, if anything, will come of "Greater Doppelgangers" in 4E? Will they retain the extraterrestrial appearance of previous editions? Will they have psionic abilities? I think such a creature could make a great Controller, Skirmisher, or Lurker monster.